The present invention relates to a method of heat treating a layered amorphous soft magnetic (ASM) film with two or more layers. More particularly, the present invention relates to a method of heat treating an ASM film in such a way that it will provide a sufficiently high permeability over a broad range of frequencies to make it suitable for use in various components that utilize magnetism such as magnetic cores in thin-film heads.
Metals in their solid state normally occur in a crystalline form in which the atoms arrange themselves in an ordered, repeating, three-dimensional pattern. However, if solutions of certain alloys are rapidly quenched from their molten state to solidify them or if certain target materials are ion-sputtered and the scattered atoms deposited on a rapidly chilled substrate, ASM materials can be produced that are solid yet have an atomic arrangement similar to that occurring in the liquid state. Such ASM materials, which are well known in the art, do not have the long-range order of the crystal structure that is present in crystalline materials. Rather. ASM materials have a random atomic arrangement which makes them inherently free from the magnetocrystalline anisotropy of crystalline materials.
However, ASM materials are prone to magnetic anisotropy that is frequently induced during their manufacture. The induced magnetic anisotropy has an uneven distribution of magnitude and direction and the magnetic characteristics of such newly prepared materials are generally poor. In addition, the ASM materials having induced magnetic anisotropy are thermally labile. Another problem associated with ASM materials is that the various process-dependent strains that develop in them during the creation of an amorphous state will remain in the bulk of the material and this will not only impair its magnetic characteristics but also make it thermally unstable. If a heat treatment is conducted after molding as in the case of manufacturing a magnetic head in which the pole material is molded in glass or some other material, the internal strain can be eliminated. However, stress will develop at the interface between the pole material and the glass. The magnetic characteristics of the pole material will thus be impaired rather than improved.
In order to remove the induced magnetic anisotropy and internal strain that have developed during the manufacture of ASM materials, various heat treatments have been proposed. Among these proposals is a method which consists of heat treating an ASM material in a rotating magnetic field in a non-oxidizing atmosphere at a temperature below the Curie point and the crystallization temperature. This method is an effective approach and is capable of improving the permeability of the material in a dc electric field or in a low-frequency range. However, if the induced magnetic anisotropy is eliminated or reduced, the domain structure of the material becomes so unstable and coarse that the domain walls tend to move easily and the permeability of the material in the high-frequency range (.gtoreq.1 MHz) is decreased.
To improve the permeability in the high-frequency range, the magnetization process must rely on the rotation of magnetization which provides a higher switching speed, rather than on the movement of domain walls. To this end, the magnetization must be driven from the axis, i.e., direction of relatively high permeability, into the directions of relatively low permeability by imparting an appropriate magnitude of uniaxial anisotropy to the magnetic material. It is also necessary to remove the residual internal strain from the magnetic material before it is covered with a dissimilar material such as glass so as to ensure the eventual removal of all strain from the bulk of the magnetic material.